AIDS (Acquired immunodeficiency syndrome), caused by human immunodeficiency virus (HIV), is one of the world's most serious health problems. 3'-Deoxy-3'-azidothymidine (AZT), which is available in the clinic, has been proven to improve the clinical and immunological status of patients with AIDS and AIDS-related complex. However, serious side effects such as anemia and leukopenia strongly limit its clinical usefulness. Although 2',3'-dideoxyinosine (DDI) and 2',3'-dideoxycytidine (DDC) have more recently been approved for the patients who do not tolerate AZT, they are also suffering from side effects such as peripheral neuropathy and pancreatitis. Therfore, there is an urgent need to develop a substance possessing higher antiviral activity and lower toxicity to the host cells. Various pyrimidine acyclonucleoside derivatives having (substituted) phenylthio group or (substituted) benzyl group at the 6-position of the pyrimidine ring have been disclosed and found to have effective antiviral activity against retrovirus (WO 89/09213, EP 420,763 A2, EP 449,726 A1). A few 6-phenylselenenyl substituted pyrimidine acyclonucleoside derivatives (J. Med. Chem. 1991, 34, 3305-3309, Antiviral Chem. & Chemother. 1992, 3(5), 263-266 and J. Heterocyclic Chem. 1194, 31, 177-185) have been synthesized, however, the antiviral activity against retrovirus is only marginal. The present inventors have synthesized a wide variety of novel pyrimidine acyclonucleoside derivatives having ethyl group or isopropyl group at the 5-position and having (substituted) phenylselenenyl group at the 6-position of the pyrimidine ring, and found that most of these pyrimidine acyclonucleoside derivatives possessed excellent antiretroviral activity to satisfy the above demand(KR Application No. 94-3794, 94-18324 and 94-18325). The present invention has been accomplishied based on this finding.